Editorial: Resistance to Salinity and Water Scarcity in Higher Plants. Insights From Extremophiles and Stress-Adapted Plants: Tools, Discoveries and Future Prospects

摘要

Abiotic stresses, such as excessive salinity and low water availability, have always presented major barriers to achieving high biomass and full yield potential in crops. Global climate change, with attendant increases in the severity of these abiotic stresses, will require directed adaptations of crop species on an unprecedented scale in order to sustain agricultural productivity. Current, and rapidly expanding, information on genome structure and function, primarily, but not exclusively, in model angiosperms, provides a starting point for a heightened understanding of genomic responses to drought and/or salt stress, within and across species. A crucial component that is leading to an acceleration in our understanding of stress responses is the ability to sequence whole genomes more rapidly and relatively inexpensively, and to begin to interpret the data that are accumulating, although such analyses are still very much in the beginning stages. Superior stress tolerance may be due to the nature of defense-related protein-coding genes, expanded gene families of stress proteins, or the “stress-readiness” of tolerant species/ecotypes. This last mechanism—stress-readiness—may be due to stress-independent, constitutively higher expression of key “defense” genes through primed signaling networks or due to specific transcription factors, the role of post-transcriptional processes, such as alternative splicing, and combinations thereof (Haak et al., 2018 ). Lest this field fall into the trap of the “drunk looking under the streetlight for his keys” (that is, searching in easily studied areas), we must keep in mind that the discovery of novel mechanisms, novel gene family members, and novel signaling pathways is a distinct possibility as the combined analytical power of sequencing and data mining increases. Filichkin et al. have conducted a comprehensive study, using both RNA-Seq and Iso-Seq, of alternative splicing events that occur in response to drought, heat and temperature stress in root, xylem and leaf tissue of poplar. They identified both stress-responsive isoforms that responded to each of the stresses that were tested, and isoforms that were unique to each of the three abiotic stresses. Interestingly, extensive stress-induced intron retention was detected in the data set, with the imposition of specific stresses being associated, in some cases, with an increase in fully spliced mRNAs, at the expense of isoforms that had retained introns, or the converse. Tissue specificity with respect to splicing events was also found. Isoforms encoding regulatory proteins showed differential intron retention (DIR), including some known poplar splicing factors. Additionally, co-regulation of DIR events occurred as a result of stress imposition. Non-coding mRNAs were also spliced, which brings the question of whether the differential splicing aims to enlarge the repertoire of cellular proteins or the physical presence of introns in the genome promotes survival under stress conditions by alternative mechanisms, as recently shown for the starvation response of the yeast Saccharomyces cerevisiae (Parenteau et al., 2019 ). Potentially, much more information lies in this large and very valuable dataset concerning the role of specific splice variants in known and novel stress response pathways. The data set is available to the public at Poplar Interactome GBrowse. In another molecular study in this special topic, Mo et al. studied cassava ( Manihot esculenta ), which is an important staple food for around 800 million people. It is tolerant to environmental stresses such as drought and heat. Specifically, the authors cloned gene family members from the CBLs and CIPKs (calcineurin B-like proteins and CBL-interacting protein kinases, respectively, which are known to be involved in calcium-mediated stress signaling) and examined their expression patterns in response to different environmental stresses. The authors also tested whether pairs of the 8 CBLs and 25 CIPKs cloned for yeast two-hybrid analysis could interact in a yeast two-hybrid system. The authors also elegantly used a yeast-based system to reconstitute a functional SOS pathway (Quintero et al., 2002 ) and to show that co-expression of the cassava proteins SOS1, CBL10, and CIPK24 restored salt tolerance in transgenic yeast. These data recall previous findings in A. thaliana (Quan et al., 2007 ) and provide further evidence of the high degree of evolutionary conservation in stress-related signaling across plant species. In the case of species that are closely related to mesophytes (plants that do not possess specific adaptation mechanisms for survival in extreme conditions) or ecotypes within species, their relatedness to better-studied close relatives increases the likelihood of discovering the genetic bases for their relatively superior stress resistance. Thus, these species or ecotypes present a rich potential resource for the identification of genes, processes, and pa